Partial Discharge (PD) phenomena are defined by IEC 60270 as localized dielectric breakdowns of a small portion of a solid or liquid electrical insulation system under high voltage (HV) stress. PD can occur, for example, in voids within solid insulation, across the surface of insulating material due to contaminants, voids or irregularities in the insulating material, within gas bubbles in liquid insulation, from metal debris in the insulating material, or around an electrode in gas (corona activity). PD causes the insulation to deteriorate progressively and can lead to electrical breakdown. The ultimate failure of HV assets due to electrical breakdown is often sudden, catastrophic and resulting in major damage and network outages.
Periodic PD off-line spot testing of the HV equipment in Electrical Substations (ES) has been used to provide a long term trending of PD activity. However, these spot tests can sometimes fail to identify failures due to changing conditions, which are related to electrical loads, environmental conditions such as temperature or humidity, or conditions related to equipment duty cycle, or seasonal related insulation problems.
In contrast to off-line testing, on-line PD testing and monitoring gives an accurate picture of the health and performance of the HV equipment in the ES, under normal service conditions including the effect of load, temperature and humidity.
The continuous monitoring of PD activity in HV equipment is now accepted as an effective method to identify trends of localized damage or insulation degradation in HV equipment before failure. The occurrence of PD is detectable as one or more of the following phenomena:
(a) Electromagnetic emissions, in the form of short pulses of current and radio waves emission, light and heat.
(b) Acoustic emissions, in the audible and ultrasonic ranges.
(c) Ozone, Nitrous Oxide and other gases emitted either to the air or dissolved in the insulating liquid.
PD events can be detected by various types of sensors which can be placed in, on, or in the vicinity of HV equipment. These sensors include:
(a) a high frequency current transducer (HFCT) sensor, which is clamped around the case of the component being tested and connected to the ground;
(b) ultra-High Frequency (UHF) internal sensor, which measures PD activity in the form of pulses of UHF radio waves;
(c) transient Earth Voltage (TEV) sensor, which measures induced voltage spikes on the surface of the metal surrounding the HV component;
(d) ultrasonic (US) sensor which measures the wide band sound waves created by the mechanical shock wave associated with the PD event. Ultrasonic sensors can be positioned in the interior or the exterior of the component under examination; and
(e) chemical sensors can detect the breakdown of HV equipment material into its chemical components due to a PD event. The two primary chemical tests employed are Dissolved Gas Analysis (DGA), and High Performance Liquid Chromatography (HPLC).
The sensors for PD detection described above require multiple connections to the HV equipment. All the technologies require at least one sensor per HV component and some of the sensors are required to be internally located in the HV equipment (see “Recent trends in the condition monitoring of transformers: Theory implementation and analysis” by: Sivaji Chakravorti; Debangshu Dey; Biswendu Chatterjee London: Springer, 2013). An alternative approach, which does not require internal placement of sensors, is a noncontact, remote-sensing technology, such as the detection of radio-frequency (RF) radiation emissions emitted during a PD event (see Moore P. J., Portugues I., Glover I. A., “A nonintrusive partial discharge measurement system based on RF technology” Power Engineering Society General Meeting, 2003, IEEE (Volume: 2)).
The RF radiation from a PD event consists of several individual high-energy, wideband pulses ranging in length from a few nanoseconds to a few microseconds. The RF radiation occurs because once a discharge is initiated, the electrons, which comprise the current of the HV equipment, are quickly depleted in the created gap, either by striking the point electrode or by attachment to gas-phase molecules. The rise time of the resulting PD pulse is sufficiently fast to extend into the RF spectrum and cause the electrically attached, supporting structures such as bus-bars, bushings, etc. to emit the impulse response RF radiation. The resulting pulses are localized, and depending on the pulses magnitude, can be readily measured within typical distances of 100 to 200 meters. The pulses of the RF radiation may be detected by a continuous RF monitoring system in the range of 500-2500 MHz comprised of an antenna array and RF receivers which are located within the ES.
The ES environment is particularly onerous for RF radiation detection. RF noise, which includes RF pulses, is generated by a wide range of energized equipment which contains stressed insulation. The main challenge of an RF based PD sensing system is to distinguish between RF noise, like operating switchgear, circuit breakers and voltage switches, and PD phenomena (see Moore P. J., Portugues I., Glover I. A., “Partial Discharge Investigation of a Power Transformer Using Wireless Wideband Radio-frequency Measurements.” IEEE Transactions on Power Delivery. Vol. 21. 2006. pp. 528-530). In RF detection of the PD signal, the localization of the faulted HV element within the ES is essential. Previous work treated the localization problem as free space localization and employed standard triangulation methods (see Moore P. J., Portugues I., Glover I. A., “RF based discharge early warning system for air-insulated substations.” IEEE Transactions on Power Delivery. Vol. 24. 2009. pp 20-29). In this work, errors in the range of several meters have been reported. Since the area of the Electrical substation (ES) contains a large amount of high power and high voltage electrical equipment, the area is saturated with a wideband electromagnetic noise. The wideband electromagnetic noise reduces the Signal to Noise Ratio (SNR), and obscures the detection of PD signals, which results in the large location errors.
In addition to the electromagnetic noise, the PD signal can be reflected, attenuated, scattered, and absorbed by the different metallic structures of the high voltage equipment. The distortions in the PD signal and the environmental ES's electromagnetic noise hinder the detection and localization of the PD source.
An additional approach to PD detection is described in U.S. Pat. No. 7,467,049. Hence a system and method for improving the localization accuracy and detection SNR by overcoming the electromagnetic noises and the distortions in the PD signal due to high power and high voltage equipment is required.